1. Field of the Invention
The present invention relates to a transmitter having an automatic power controller which switches the output power of a mobile communication terminal, cellular telephone set, or the like in a step-wise fashion by an external control signal so as to hold the same constant.
A transmitter functioning as a mobile communication terminal is not used under continuously constant conditions. It may be used, for example, somewhere close to a communication base station or somewhere outside of a service area far from the communication base station. Further, it may be used at high temperature locations or low temperature locations. In addition, when the transmitter is driven using a car battery as its power source, it is influenced by the fluctuations in the voltage of the battery.
Therefore, an automatic power controller (APC) is essential for a transmitter functioning as a terminal for, for example, mobile communication. Further, it is usually desirable to be able to switch the output power of the transmitter in a number of steps so as to adjust the same.
2. Description of the Related Art
As will be explained in detail later, the conventional transmitter with an APC is mainly comprised of a radio frequency power amplifier which amplifies a radio frequency input signal by a gain determined in accordance with a control voltage; a first means which branches and receives a radio frequency output signal from the radio frequency power amplifier, produces a voltage proportional to the deviation of the received voltage from a threshold voltage, and gives feedback to the radio frequency power amplifier using this as the control voltage; and a second means which generates an offset voltage to be superposed on the threshold voltage in accordance with an external command, wherein the second means is comprised of a multiplexer which outputs the offset voltage; series resistors comprised of a group of constant resistors which supply a variety of types of offset voltages to the multiplexer; and a variable resistor which is connected to one end of the series resistors. The variable resistor is for fine adjustment of the offset voltage at the time of factory shipment of the transmitter.
The above-mentioned conventional transmitter with an APC, however, had the following problem:
i) Assume there is a desire to finely adjust one of the offset voltages among the plurality of offset voltages obtainable from the intermediate connecting points of the series resistors. The resistance value of the variable resistor is finely adjusted to obtain this desired offset voltage. Even if the offset voltage has become the desired amplitude, however, since the resistance value of the variable resistor was changed, all the other offset voltages end up slightly deviating from the optimum voltage values. This is the first problem.
ii) After the variable resistor is adjusted manually at the time of factory shipment, it is not easy for the user to adjust the same. Therefore, it is not possible to adjust the offset voltages during use of the transmitter. This is the second problem.
In addition, as mentioned in i) above, during the operation of the transmitter, it is not possible to adjust the offset voltages independently from each other.
iii) A conventional APC is comprised of a group of a plurality of constant resistors and a single variable resistor, as mentioned earlier. Therefore, there is the third problem that it is difficult to make the resistors by an integrated circuit (IC) when fabricating the internal circuits of the transmitter in an IC. This proves to be a hindrance when trying to reduce the size of the transmitter.